A heater element that has an NTC of resistance will decrease in resistance as it heats up. Carbon based heater elements, such as graphite and carbon fiber heaters, have an NTC of resistance and, thus, can be referred to as NTC heater elements.
In heater temperature control, the thermal conductivity of a heated substrate or object is almost always relied upon to pass thermal energy to a sensor or thermostat. When the thermal conductivity is low, a delayed response is often experienced. This delay can result in catastrophic failure of the heater. A similar delay can be the result of improper mounting of the heater element or the use of the heater element for an improper application. For example, if the heater element is not held or adhered securely to the object/material to be heated, the effective thermal conductivity can be extremely low, even if the materials have a high thermal conductivity. In this case, the “effective thermal conductivity” can be defined as the material's thermal conductivity plus the thermal contact conductivity or the conductivity across the interface between the heater and the heated object/material. Often, due to thermal expansion or aging materials, the thermal transfer efficiency degrades over time. Eventually, the temperature climbs to an often dangerous level. The present invention, however, can help to prevent this temperature increase.
The thermal lag mentioned above can also cause a great deal of hysteresis about a set temperature. Often the solution to this type of problem is to use sophisticated temperature controls which use pulse-width-modulation or variable voltage to hold a temperature steady. On the other hand, the present invention can achieve tight temperature control of the heater element using a much simpler On-Off methodology, since the heat source can be held at a near constant temperature due to little to no delay in temperature sensing. The present invention can also more accurately deal with variable thermal loads, since the heat is controlled from the source.